1. Field of the Invention
The invention relates to a shaft furnace and more particularly to the tap hole structure of a shaft furnace.
2. Description of the Prior Art
Dutch patent application no. 8301862 describes a metallurgical shaft furnace, e.g. a blast furnace, comprising a steel outer shell and a refractory lining which is fitted inside and against the steel outer shell. The furnace has at least a tap hole structure which is composed of a steel sleeve, also called a cooling box, fitted on the outer shell, within which sleeve a permanent lining is fitted. Within and against the permanent lining is a refractory sealing material at the location where the tap hole is to be formed. The permanent lining and the sealing material lie directly against the refractory lining of the shell. The sealing material may be refractory bricks or a refractory compressed compound, for sealing off the tap hole.
A problem with this known shaft furnace is that it is liable to gas leaks, specifically carbon monoxide gas coming out of the furnace at the tap hole. In order to avoid this gas leakage, in a known structure illustrated by FIGS. 1 to 3 of the accompanying drawings and described in detail below, the refractory lining of the furnace in the surroundings of the tap hole is provided with a double cage structure with copper sheets. However, the sealing which can be achieved in this way is not such that gas leakage is fully prevented. In some cases the gas leakage may still be so great that gas discharging from the closed tap hole ignites directly after escaping from the furnace. This is an unsafe and undesirable situation. But even if the discharging gas does not burn, the situation may still be unsafe given that the discharging gas is rich in carbon monoxide which makes work in the surroundings of the tap hole impossible to carry out because of the danger of poisoning, or possible only when carried out under inconvenient conditions (with gas masks). Furthermore there is another inconvenience. The escaping CO gas affects the binder of the rammed compound so that the service life of the tap hole sealing is limited.
Japanese liad-open patent application no. JP-A-52-7308 (1977) discloses a method of preventing gas leakage at the tap hole by applying a temporary metal box at the exterior of the refractory lining and injecting refractory material to seal the lining.